Spanish moss is actually a bromeliad (pineapple relative) and is epiphytic, not parasitic. It draws no nutrition or water from the tree and, in itself, is not harmful. However, if a tree is stressed from environmental pressures such as drought, insect, or disease, the amount of foliage will be reduced permitting more light to penetrate the canopy and allowing the Spanish moss to multiply. It would be wise to inspect the overall status of the tree (or employ a certified arborist) to determine what might be causing the decline. There are no chemical products labeled for removing the moss, but an extension pole might be helpful in controlling unwanted quantities. 

Additional Resource 

http://gardeningsolutions.ifas.ufl.edu/giam/shows/transcripts/2007/april_2007/spanish_moss.pdf